Natural infection with measles virus (MV) establishes lifelong immunity. Persistent infection with MV is likely involved in this phenomenon, as non-replicating protein antigens never induce such long-term immunity. Although MV establishes stable persistent infection in vitro and possibly in vivo, the mechanism by which this occurs is largely unknown. Here, we demonstrate that MV changes the infection mode from lytic to non-lytic and evades the innate immune response to establish persistent infection without viral genome mutation. We found that, in the persistent phase, the viral RNA level declined with the termination of interferon production and cell death. Our analysis of viral protein dynamics shows that during the establishment of persistent infection, the nucleoprotein level was sustained while the phosphoprotein and large protein levels declined. The ectopic expression of nucleoprotein suppressed viral replication, indicating that viral replication is self-regulated by nucleoprotein accumulation during persistent infection. The persistently infected cells were able to produce interferon in response to poly I:C stimulation, suggesting that MV does not interfere with host interferon responses in persistent infection. Our results may provide mechanistic insight into the persistent infection of this cytopathic RNA virus that induces lifelong immunity.

f5: MV N, P, and L protein levels in MV-infected B95a cells.(a) Intracellular MV N protein was stained at the indicated dpi. Open and filled histograms indicate uninfected and infected B95a cells, respectively. (b) Western blotting of MV N, P, and L proteins at the indicated dpi. GAPDH was used as an internal control. Averages of the relative protein amounts of N, P and L proteins at 23 dpi in three independent infections are graphed.

Mentions:
Next, we aimed to determine how MV restricts its own replication during persistent infection without causing any alterations in either the viral genome sequence or the host antiviral response. We investigated this by monitoring the levels of intracellular viral proteins, and we began by using a FACS analysis to examine the intracellular N protein expression because the N protein is the most abundant viral protein. We found that the level of this protein was sustained during the establishment phase of persistent MV infection (Fig. 5a). Thus, we next focused on the accumulation of other viral proteins. The accumulation of viral proteins that are required for replication was kinetically monitored by western blotting assays. The expression levels of P and L proteins each increased at 1 and 23 days after infection and then declined quickly, following the kinetics of vRNA levels (Figs 3a and 5b). The N protein level was sustained for a longer period than the levels of P and L proteins and vRNA (Figs 3a and 5b).

f5: MV N, P, and L protein levels in MV-infected B95a cells.(a) Intracellular MV N protein was stained at the indicated dpi. Open and filled histograms indicate uninfected and infected B95a cells, respectively. (b) Western blotting of MV N, P, and L proteins at the indicated dpi. GAPDH was used as an internal control. Averages of the relative protein amounts of N, P and L proteins at 23 dpi in three independent infections are graphed.

Mentions:
Next, we aimed to determine how MV restricts its own replication during persistent infection without causing any alterations in either the viral genome sequence or the host antiviral response. We investigated this by monitoring the levels of intracellular viral proteins, and we began by using a FACS analysis to examine the intracellular N protein expression because the N protein is the most abundant viral protein. We found that the level of this protein was sustained during the establishment phase of persistent MV infection (Fig. 5a). Thus, we next focused on the accumulation of other viral proteins. The accumulation of viral proteins that are required for replication was kinetically monitored by western blotting assays. The expression levels of P and L proteins each increased at 1 and 23 days after infection and then declined quickly, following the kinetics of vRNA levels (Figs 3a and 5b). The N protein level was sustained for a longer period than the levels of P and L proteins and vRNA (Figs 3a and 5b).

Natural infection with measles virus (MV) establishes lifelong immunity. Persistent infection with MV is likely involved in this phenomenon, as non-replicating protein antigens never induce such long-term immunity. Although MV establishes stable persistent infection in vitro and possibly in vivo, the mechanism by which this occurs is largely unknown. Here, we demonstrate that MV changes the infection mode from lytic to non-lytic and evades the innate immune response to establish persistent infection without viral genome mutation. We found that, in the persistent phase, the viral RNA level declined with the termination of interferon production and cell death. Our analysis of viral protein dynamics shows that during the establishment of persistent infection, the nucleoprotein level was sustained while the phosphoprotein and large protein levels declined. The ectopic expression of nucleoprotein suppressed viral replication, indicating that viral replication is self-regulated by nucleoprotein accumulation during persistent infection. The persistently infected cells were able to produce interferon in response to poly I:C stimulation, suggesting that MV does not interfere with host interferon responses in persistent infection. Our results may provide mechanistic insight into the persistent infection of this cytopathic RNA virus that induces lifelong immunity.